This invention relates to a liquid epoxy resin composition which cures into a cured product having improved adhesion to the surface of silicon chips and especially photosensitive polyimide resins and nitride films, high humidity resistance, and stability against thermal shocks. It also relates to a semiconductor device which is sealed with the cured product of the liquid epoxy resin composition.
The trend toward smaller sizes, lighter weights and increased capabilities in electrical equipment has led to a shift in the dominant semiconductor mounting process from pin insertion to surface mounting. Progress of semiconductor devices toward a higher degree of integration entails the enlargement of dies having a size as large as 10 mm or more per side. For semiconductor devices using such large size dies, attention is paid to the problems that greater stresses are applied to the die and the sealant during solder reflow, separation occurs at the interface between the sealant and the die or substrate, and the package cracks upon substrate mounting.
Liquid epoxy resin compositions which are the common prior art sealants are less adherent to the surface of silicon chips and especially photosensitive polyimide resins and nitride films. In the semiconductor art, there remains a need for improving adhesion to such materials. Under the circumstances, phthalic anhydrides and analogous acid anhydrides are often used as the curing agent in liquid epoxy resin compositions. However, the phthalic anhydrides and analogues are hygroscopic and suffer from several problems. Prior to curing, a phenomenon occurs that the penetration varies or even ceases midway due to a viscosity rise resulting from moisture absorption. The prior art acid anhydrides readily take up water prior to curing, and promote hydrolysis even after curing. Such moisture absorption causes a volume expansion, which can increase the electrical resistance at the interface between solder bumps and leads in flip-chip type semiconductor devices, giving rise to a reliability problem.
An object of the invention is to provide a liquid epoxy resin composition which cures into a cured product that has improved adhesion to the surface of silicon chips and especially photosensitive polyimide resins and nitride films, does not deteriorate under hot humid conditions as encountered in PCT (120xc2x0 C./2.1 atm), and does not peel or crack over several hundred cycles of thermal cycling between xe2x88x9265xc2x0 C. and 150xc2x0 C. Another object of the invention is to provide a semiconductor device which is sealed with the cured product of the liquid epoxy resin composition.
The invention pertains to a liquid epoxy resin composition comprising (A) a liquid epoxy resin, (B) a curing agent, (C) a curing accelerator, and (D) an inorganic filler. We have been found that by blending as the curing agent (B) a mixture of 3,4-dimethyl-6-(2-methyl-1-propenyl)-1,2,3,6-tetrahydrophthalic acid and 1-isopropyl-4-methyl-bicyclo[2.2.2]oct-5-ene-2,3-dicarboxylic acid in an amount of 5 to 75 parts by weight per 100 parts by weight of the entire curing agent, there is obtained a liquid epoxy resin composition that is effectively adherent to the surface of silicon chips and especially photosensitive polyimide resins and nitride films, remains intact under hot humid conditions as encountered in PCT (120xc2x0 C./2.1 atm), and is fully resistant to thermal shocks, so that it is suited as a sealant for large die size semiconductor devices.
Particularly, 3,4-dimethyl-6-(2-methyl-1-propenyl)-1,2,3,6-tetrahydrophthalic acid and 1-isopropyl-4-methyl-bicyclo[2.2.2]oct-5-ene-2,3-dicarboxylic acid are per se known as the curing agent for liquid epoxy resins. However, these compounds are used as the curing agent in few liquid epoxy resin compositions because they, when used alone, provide epoxy resin compositions with a high viscosity and give cured products which are brittle. Quite unexpectedly, we have found that when the curing agent is formulated by blending a mixture of 3,4-dimethyl-6-(2-methyl-1-propenyl)-1,2,3,6-tetrahydrophthalic acid and 1-isopropyl-4-methyl-bicyclo[2.2.2]oct-5-ene-2,3-dicarboxylic acid with another curing component in a specific proportion, and preferably selecting the other curing component from acid anhydrides that ensures formation of tough cured products, the resulting liquid epoxy resin composition is dramatically improved in adhesion to the silicon chip surface and thermal shock resistance, exhibits satisfactory properties even under hot humid conditions, and is best suited as a sealant for large die size semiconductor devices.
Accordingly, the invention provides a liquid epoxy resin composition comprising (A) a liquid epoxy resin, (B) a curing agent, (C) a curing accelerator, and (D) an inorganic filler as essential components. The curing agent (B) contains 5 to 75 parts by weight of a mixture of 3,4-dimethyl-6-(2-methyl-1-propenyl)-1,2,3,6-tetrahydrophthalic acid and 1-isopropyl-4-methyl-bicyclo[2.2.2]oct-5-ene-2,3-dicarboxylic acid per 100 parts by weight of the entire curing agent.